1. Field of the Invention
The present invention relates to a heterocyclic compound that can be used as a light-emitting element material. The present invention relates to a light-emitting element, a display module, a lighting module, a light-emitting device, a display device, a lighting device, and an electronic device each using the heterocyclic compound.
2. Description of the Related Art
As next generation lighting devices or display devices, display devices using light-emitting elements (organic EL elements) in which organic compounds are used as light-emitting substances have been rapidly developed because of their advantages of thinness, lightweightness, high speed response to input signals, low power consumption, and the like.
In an organic EL element, voltage application between electrodes, between which a light-emitting layer is interposed, causes recombination of electrons and holes injected from the electrodes, which brings a light-emitting substance into an excited state, and the return from the excited state to the ground state is accompanied by light emission. Since the wavelength of light emitted from a light-emitting substance depends on the light-emitting substance, use of different types of organic compounds as light-emitting substances makes it possible to obtain light-emitting elements which exhibit various wavelengths, i.e., various colors.
In the case of display devices which are used to display images, such as displays, at least three-color light, i.e., red light, green light, and blue light is necessary for reproduction of full-color images. Furthermore, in application to lighting devices, it is ideal to obtain light thoroughly covering the visible light region for obtaining a high color rendering property, but in reality, light obtained by mixing two or more kinds of light having different wavelengths is often used for lighting application in many cases. It is known that, with a mixture of three-color light, i.e., red light, green light, and blue light, white light having a high color rendering property can be obtained.
Color of light emitted from a light-emitting substance depends on the substance, as described above. However, important performances as a light-emitting element, such as lifetime, power consumption, and emission efficiency, are not only dependent on a light-emitting substance but also greatly dependent on the materials in the layers other than the light-emitting layer, an element structure, and the like. Therefore, many kinds of materials for light-emitting elements are necessary for the growth of this field. For the above-described reasons, materials for light-emitting elements with a variety of molecular structures have been proposed (e.g., see Patent Document 1).
As is generally known, the generation ratio of a singlet excited state to a triplet excited state in a light-emitting element using electroluminescence is 1:3. Therefore, a light-emitting element in which a phosphorescent material capable of converting the triplet excited state to light emission is used as a light-emitting substance can theoretically obtain higher emission efficiency than a light-emitting element in which a fluorescent material capable of converting the singlet excited state to light emission is used as a light-emitting substance.
However, since the triplet excited state of a substance is at a lower energy level than its singlet excited state, a substance that emits phosphorescence has a higher singlet excited state than a substance that emits fluorescence when the emissions of the substances are at the same wavelength.
As a host material in a host-guest type light-emitting layer or a substance contained in a transport layer in contact with a light-emitting layer, a substance having a wider band gap or a higher triplet level (T1, an energy difference between a triplet excited state and a singlet ground state) than a light-emitting substance is used for efficient conversion of excitation energy into light emission from the light-emitting substance.
Therefore, a host material and a carrier-transport material each having a further wider band gap are necessary in order to efficiently obtain phosphorescence. It is very difficult to develop a substance which has a wide band gap while enabling the production of a light-emitting element, with a low driving voltage and a high emission efficiency.